revise text about column ordering

[grahamegrieve]

Column ordering: ViewDefinition runners MUST produce columns in the same order as they appear in the views.

I think 'views' isn't the right word in that sentence.

Also, given that not all runners produce ordered sets of columns, I think that this needs some qualification

[johngrimes]

The text has changed since this issue was created, it now reads:

ViewDefinition runners MUST produce output in the same order as the column structures seen in the ViewDefinition instances.

The other perhaps pertinent change has been to require column names within view definitions.

In the context of this, why is it desirable to require deterministic column ordering in the output?

[johngrimes]

I think this has now been resolved by #208.